Hey Tray, you will need to make sure that you have the X-Forwarded-For header passed through. The filter will look for the existence of that header and then replace the "c-ip" IIS log value with that supplied HTTP header. Depending on the version of BIG-IP you are using, this should be an option in the virtual server. If not, then you'll have to write an iRule to inject that header. Let me know if you need to go that route and I'll help you out.
-Joe